As demands for VLSI devices having an increased density and improved precision are further increasing in recent years, EUV lithography, which is an exposure technique using an extreme ultraviolet light (hereinafter, referred to as “EUV”), is considered promising. The EUV light indicates light in a wavelength range of a soft X-ray region or a vacuum ultraviolet region, specifically, light having a wavelength of about 0.2 to 100 nm.
A reflective mask used in such lithography has formed on, e.g., a glass or silicon substrate a multilayer reflective film for reflecting an exposure light, wherein the multilayer reflective film has formed therein a pattern of an absorber film for absorbing an exposure light. In an exposure machine for performing pattern transfer, light striking the reflective mask mounted on the exposure machine is absorbed by portions of the multilayer reflective film having the absorber film pattern and reflected by portions of the multilayer reflective film having no absorber film pattern. Then, the reflected light image is transferred through a reflection optical system onto a semiconductor substrate, such as a silicon wafer.
For achieving a semiconductor device having an increased density and improved precision using the above reflective mask, the reflective region in the reflective mask (the surface of the multilayer reflective film) is required to have a high reflectance with respect to an EUV light, which is an exposure light.
To achieve the above-described high reflectance, the multilayer reflective film is a multilayer film comprising elements having different refractive indices, which are periodically laminated, and, generally, a multilayer film is used in which a thin film of a heavy element, or a compound thereof, and a thin film of a light element, or a compound thereof, are alternately laminated in about 40 to 60 cycles of the layers. For example, as a multilayer reflective film for an EUV light having a wavelength of 13 to 14 nm, a Mo/Si periodically laminated film, in which a Mo film and a Si film are alternately laminated in about 40 cycles of the layers, is preferably used. Mo is easily oxidized in air which decrease the reflectance of the multilayer reflective film, and therefore the Si film constitutes the uppermost layer of the multilayer reflective film.
As a reflective mask used in the EUV lithography, for example, there is a reflective mask for exposure described in Patent Document 1 below. Specifically, Patent Document 1 has proposed a reflective photomask characterized by having: a substrate; a reflective layer, formed on the substrate, comprising a multilayer film in which two different films are alternately laminated; a buffer layer, formed on the reflective layer, comprising a ruthenium film; and an absorber pattern, formed on the buffer layer so as to have a predetermined pattern form, comprising a material capable of absorbing a soft X-ray.
The above-described buffer layer is also called a protective film. When forming the absorber pattern, a part of the absorber film is etched through a resist, and, to ensure the formation of the absorber pattern, the absorber film is subjected to over etching slightly, and therefore the film present under the absorber film is inevitably etched. In this instance, to prevent the multilayer reflective film under the absorber film from suffering a damage, a protective film is formed.
With respect to the protective film, further, from the viewpoint of suppressing the formation of a diffused layer (which leads to a reduction of the reflectance of the multilayer reflective film) between the Si layer constituting the surface layer of the multilayer reflective film and the protective film, a protective film comprising a Ru alloy having Zr or B added to Ru has been proposed (Patent Document 2).
Further, Patent Document 3 has proposed a method of manufacturing an EUV mask blank, which can reduce foreign matter caused due to a sputtering target, in which a Mo film and a Si film are alternately deposited on a substrate to form a multilayer reflective film, and a Ru film or a Ru compound film is deposited on the multilayer reflective film, wherein the deposition of these films is conducted by an ion beam sputtering method in the same deposition chamber under predetermined conditions. In this document, examples of the above-described Ru compounds, include RuB, RuNb, and RuZr.